Announced during Nintendo's keynote at the Game Developer's Conference of 2006, very little is know about Phantom Hourglass. We know only what we have deduced from the trailer:

It uses celshading, in a style identical to the Wind Waker's. The DS does it admirably, at a framerate that seems to be equal to the Wind Waker's. Of course, the graphical quality is scaled back a bit, but it still impressive.

It utilizes the touchscreen in several ways. The trailer shows the player drawing a route on the screen, and then throwing the boomerang - which follows the route drawn. At the beginning of the trailer, the touchscreen is used to trace a picture of an hourglass on a door, which then opens. Finally, at one point it shows a route being traced on a map of an ocean, and then it looks like Link's boat automatically follows the route.

It is probably a safe bet that it takes place after the Wind Waker, but this has not been confirmed.

We know from the trailer that it will take place at least in part on an ocean, and Link has a steamboat.

The crystal switches from A Link to the Past, Link's Awakening, and other 2D games are making a return.